1. Field of the Invention
The present invention relates to a LED driver circuit, and more particularly to a LED driver circuit, of which at least one bias current is drawn from a load current flowing out of a LED module, and a low voltage controller can be used for controlling a switching operation to deliver the load current.
2. Description of the Related Art
FIG. 1 shows the architecture of a prior art LED driver circuit. As shown in FIG. 1, the architecture includes: a LED driver 100, an NMOS transistor 101, a resistor 102, a LED module 103, an inductor 104, and a diode 105.
In the architecture of the prior art LED driver circuit, the LED driver 100, drawing a bias current Ibias from an DC voltage source VIN—for example 127V, is used for generating a PWM signal VG with a duty cycle in response to a current sensing voltage VCS.
The NMOS transistor 101 is used to control the magnetic flux transformation through the inductor 104 in response to the PWM signal VG. When the NMOS transistor 101 is during a turn-on period, the LED module 103, the inductor 104, the NMOS transistor 101, and the resistor 102 will constitute a conduction path to store the magnetic flux in the inductor 104; when the NMOS transistor 101 is during a turn-off period, a conduction path composed of the LED module 103, the inductor 104, and the diode 105 will be formed to release the magnetic flux from the inductor 104.
The resistor 102, connected between the NMOS transistor 101 and a reference ground, is used for converting the current, which corresponds to the magnetic flux being stored in the inductor 104, to the current sensing voltage VCS when the NMOS transistor 101 is during a turn-on period.
The LED module 103 is the load of the LED driver circuit.
The inductor 104 is used for carrying the magnetic flux to provide a current to drive the LED module 103.
The diode 105 is used for releasing the magnetic flux in the inductor 104 to drive the LED module 103.
Through a periodic on-and-off switching of the NMOS transistor 101, which is driven by the PWM signal VG generated from the LED driver 100, the input power from the voltage source VIN is transformed through the inductor 104 to the LED module 103 in the form of a regulated current.
However, there are disadvantages in the prior circuit of FIG. 1. First, the LED driver 100 has to be a high voltage controller which is more expensive than a low voltage one. Second, the bias current Ibias for the LED driver 100 causes additional power consumption. Third, if the LED module 103 is open circuited due to some abnormal situations, the NMOS transistor 101 will be kept turned on by the LED driver 100, and most of the voltage dropt of the voltage source VIN will be across the diode 105. As such, a substantial leakage current is produced.
Therefore, there is a need to provide a LED driver circuit which is more power saving and cost effective.
Seeing this bottleneck, the present invention proposes a novel LED driver circuit, providing at least one bias current from a load current flowing out of a LED module, and allowing a low voltage controller for controlling a switching operation to deliver the load current.